Caption: Red blood cells. Computer artwork of human red blood cells (erythrocytes) in a blood vessel. Red blood cells are biconcave, giving them a large surface area for gas exchange, and highly elastic, enabling them to pass through narrow capillary vessels. The nucleus and other organelles are lost as the cells mature. The interior of each cell is packed with haemoglobin, a red iron-containing pigment that has an oxygen-carrying capacity. The main function of red blood cells is to distribute oxygen to body tissues and to carry waste carbon dioxide back to the lungs.